Hydrostatic transmission center section

ABSTRACT

An integrated hydrostatic transaxle having a common housing for a hydrostatic transmission and a pair of oppositely-extending differentially-connected axles to have the common housing provide a common sump therefor and with a specifically-shaped center section interconnecting the pair of hydraulic displacement units constituting the hydrostatic transmission. The center section is a unitary member separate from the common housing, with the exterior thereof exposed to the common sump. The center section has passages and arcuate ports which are interconnected by certain of said passages and which are located whereby the center section may be cast to minimize any machining required to finish the center section. Such casting can result in a porous center section with possible leakage from a passage having high pressure fluid. However, such leakage is not a problem since the leakage would not be through the wall of the common housing but to the common sump within the common housing which is at atmospheric pressure because of the center section being separate from the common housing.

DESCRIPTION

1. Technical Field

This invention pertains to a center section for a hydrostatictransmission, with the hydrostatic transmission having particularutility as a component of an integrated hydrostatic transaxle. Atransaxle of a type used in equipment, such as a lawn tractor, has gearreduction and axle components mounted in a housing providing a sump forlubricating oil. The disclosed center section is directed toward aneconomical integration of the hydrostatic transmission with thetransaxle components in a common housing providing a common sump.

2. Background Art

Hydraulically driven equipment, such as a lawn tractor, have hadtransaxle structure mounted in a housing including a drive inputconnection, a gear reduction drive, and oppositely-extendingdifferentially-connected axles, and a hydrostatic transmission isconnected to the exterior of the housing whereby a drive output from thehydrostatic transmission connects to the drive input to the transaxlestructure.

The known prior art structures have not integrated the hydrostatictransmission with the transaxle components in a common housing toprovide a common sump and with the use of a unique center sectionbetween the hydraulic components of the hydrostatic transmission asdisclosed herein.

A hydrostatic transmission has a pair of hydraulic displacement unitswith fluid connections therebetween. In a typical hydrostatictransmission, the hydraulic displacement units each have a rotatablecylinder block mounting a plurality of reciprocal pistons and with thepiston-receiving chambers in the cylinder block communicating with portsfor fluid flow to and from the piston-receiving chambers. Many differenttypes of structure are known for achieving fluid communication betweenthe arcuate ports associated with the pair of rotatable cylinder blocks.Such structure can be by means of tubing or by means of a structuralsection with fluid passages and positioned adjacent both rotatablecylinder blocks. This structural section can be either integral with ahousing for the hydrostatic transmission or a separate componentmountable between the hydraulic displacement units and separable fromthe housing.

A prior art hydrostatic transmission has a pair of hydraulicdisplacement units generally in side-by-side relation and with arotatable cylinder block of each of the hydraulic displacement unitsbeing associated with a structural section having arcuate ports forassociation with both of the hydraulic displacement units. A pair ofgenerally parallel straight passages, formed in the structural sectionintersect and communicate with the arcuate ports in pairs whereby thereis fluid communication between a pair of arcuate ports associated onewith each of the hydraulic displacement units.

The prior art also includes hydrostatic transmissions wherein thehydraulic displacement units are disposed at a selected fixed anglerelative to each other whereby the axes of rotation of the rotatablecylinder blocks thereof are at an angle to each other and a structuralsection disposed therebetween has had a pair of faces at the selectedangle whereby arcuate ports associated therewith may coact with theangularly-related cylinder blocks of the hydraulic displacement units.

DISCLOSURE OF THE INVENTION

The integrated hydrostatic transaxle disclosed herein has resulted fromefforts to reduce the cost, size and weight of a transaxle package whichhas had a non-integrated relation between the housings for a hydrostatictransmission and the gear reduction, differential and axle components.Elimination of as much machining as possible contributes substantiallyto cost reduction.

A primary feature of the invention is to provide a one-piece, generallyL-shaped center section for a hydrostatic transmission which ispositionable in a housing and has first and second faces for coactionwith rotatable cylinder blocks of a pair of hydraulic displacement unitsof the hydrostatic transmission and with the center section designed torequire a minimal amount of machining to the body thereof with resultantmaximum cost savings.

The lowest possible machining cost for the center section can beachieved by going to a casting process, such as die casting or the lostfoam process. A casting process results in a more porous center sectionand, with passages therein having fluid at high pressure, it isimportant to assure that leakage from the center section shall not be aproblem.

The one-piece generally L-shaped center section being separable from thehousing for the hydrostatic transmission and mountable therein permitscasting of the center section since leakage from a porous cast centersection will leak into a sump defined by the housing for the hydrostatictransmission, rather than through a wall of the housing.

An object of the invention is to provide, in combination, a hydrostatictransmission comprising a pair of hydraulic displacement units eachhaving a rotatable cylinder block with reciprocal pistons and a housingfor the displacement units providing a fluid sump along with a unique,one-piece, generally L-shaped center section positionable in the housingto facilitate utilization of such a structure with drive components fora hydraulically-driven device all in a common housing having a commonsump.

Additionally, the center section of the hydrostatic transmission isuniquely designed with passages in addition to first and secondgenerally straight passages interconnecting the hydraulic displacementunits to provide for mounting of bypass valves as well as delivery ofmake-up oil to the hydraulic circuit and provide for bleed of air fromthe hydraulic circuit during operation of the bypass valves.

A further object of the invention is to provide, in combination, ahydrostatic transmission comprising a pair of hydraulic displacementunits each having a rotatable cylinder block with reciprocal pistons,and a housing for said displacement units providing a fluid sump, saidrotatable cylinder blocks having their axes of rotation generally normalto each other, a one-piece generally L-shaped center sectionpositionable in said housing and having first and second faces generallyat right angles to each other, said center section being positioned tohave said first face engage an end of one rotatable cylinder block andthe second face engage an end of the other rotatable cylinder block,arcuate fluid ports at the face of each of said center section faces forcoaction with a rotatable cylinder block, a first straight fluid passagein said center section connecting one of the ports at each face todefine a pair of fluid communicating ports and terminating at one ofsaid pair of ports, and a second straight fluid passage in said centersection connecting another of the ports on each face to define a secondpair of fluid communicating ports and terminating at one of the ports ofsaid second pair.

Another feature of the invention is to provide an integrated hydrostatictransaxle having a common housing for a hydrostatic transmission and apair of oppositely-extending, drivingly-connected axles to provide acommon sump, with the hydrostatic transmission having the center sectionas described in the preceding paragraphs. Cost effectiveness is achievedby use of the common housing, common sump and one-piece center sectionwhereby leakage from the hydrostatic transmission including from a fluidpassage in the center section containing fluid pressure may reach thecommon sump at atmospheric pressure. This makes it possible to cast thecenter section and minimize costly machining even though the centersection may be more porous.

An object of the invention is to provide an integrated hydrostatictransaxle having the structure referred to in the preceding paragraph.

Still another object of the invention is to have, in combination, ahydrostatic transmission comprising a pair of hydraulic displacementunits each having a rotatable cylinder block with reciprocal pistons,and a housing for said displacement units providing a fluid sump, saidrotatable cylinder blocks having their axes of rotation normal to eachother, a one-piece L-shaped center section separate from said housingand having first and second faces at right angles to each other, saidcenter section being positioned to have said first face engage an end ofone rotatable cylinder block and the second face engage an end of theother rotatable cylinder block, each of said center section faces havingarcuate fluid ports for coaction with a rotatable cylinder block, afirst straight fluid passage in said center section connecting one ofthe ports on each face and terminating at one of said ports, a secondstraight fluid passage in said center section connecting another of theports on each face and terminating at one of said ports, said centersection being of material which may be sufficiently porous to permitleakage of high pressure fluid from whichever one of said straight fluidpassages contains high pressure fluid with said leakage flowing to saidfluid sump, and said center section having third and fourth straightfluid passages intersecting said first and second fluid passages,respectively, and opening to a surface of said center section oppositeto one of the faces thereof for mounting of check valves.

Still another object of the invention is to have the combination as setforth in the preceding paragraph wherein said first and second fluidpassages are generally parallel, said center section has a through boreextending perpendicular to and positioned between said first and secondfluid passages, a fifth fluid passage extending generally parallel toand positioned between said first and second fluid passages and openingto said bore for delivery of make-up fluid to said bore, and a sixthfluid passage extending between the fifth fluid passage and a recess setback from the surface to which the third and fourth fluid passages openfor communication with a source of filtered make-up fluid.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of the integrated hydrostatic transaxle,taken looking toward the left in FIG. 2;

FIG. 2 is a plan view of the integrated hydrostatic transaxle, withparts broken away;

FIG. 3 is a vertical section, taken generally along the line 3--3 inFIG. 2, and on an enlarged scale;

FIG. 4 is a fragmentary section of the bottom part of the housing andstructure related thereto, as shown generally along section 4--4 in FIG.3;

FIG. 5 is a fragmentary plan view of structure shown in FIG. 2;

FIG. 6 is a fragmentary section, taken generally along 6--6 in FIG. 5;

FIG. 7 is a vertical section of the center section, taken generallyalong the line 7--7 in FIG. 8 and with check valve and bypass structureshown in association therewith;

FIG. 8 is a top view of the center section for the hydrostatictransmission;

FIG. 9 is a bottom view of the center section of the hydrostatictransmission;

FIG. 10 is a side elevation of the center section, looking toward theright side thereof, as shown in FIG. 8; and

FIG. 11 is a vertical section of the center section, taken generallyalong the line 11--11 in FIG. 8 and with the structure associated withthe center section being omitted.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The integrated hydrostatic transaxle is shown generally in FIGS. 1 to 3.

The integrated hydrostatic transaxle has a common housing 10 for thecomponents thereof. The common housing 10 is of two parts, with a toppart 12 and a bottom part 14 which are joined together along a splitline 16 which is disposed generally horizontal when the integratedhydrostatic transaxle is installed in operative position. The housingparts 12 and 14 are held in assembled relation by a series of bolts 18extending through peripheral flanges of the top and bottom housing partswhich abut at the split line 16.

The shape of the housing parts in plan is shown in FIG. 2 wherein aportion of the top housing part 12 is seen in the lower left part of theFIGURE and with the remainder thereof broken away to show the bottomhousing part 14.

The common housing 10 encloses a hydrostatic transmission having a pairof hydraulic displacement units, indicated generally at 20 and 22,respectively, and also houses transaxle components, seen particularly inFIG. 2. The transaxle components include a pair of oppositely-extendingaxles 23 and 24 having ends extended beyond the bottom housing part formounting of drive wheels (not shown) and their centerlines arecoincident with the housing split line 16. The bottom housing part 14has bearings 25 and 26 at the outboard ends and thrust bearings 27 and27a at the inboard ends of the axles for rotatable support thereof andwith the axles being geared together through a differential, indicatedgenerally at 28. This differential includes bevel gears 29 and 30 at theinner end of the respective axles 23 and 24 with drive input gearsthereto including a gear 31 which meshes with an output gear 32 of agear reduction drive. The gear reduction drive has a drive inputconnection from the hydraulic displacement unit 22, with the outputshaft 35 (FIG. 3) of the latter having a gear 36 which meshes with agear 37. The latter gear is rotatably fixed to a gear 38 which mesheswith the previously-mentioned gear 32.

A brake for the drive is mounted externally of the common housing 10 andassociated with an end of the drive output shaft 35, with this brakestructure, including a brake 40, a brake drum 41 and a brake cover 42.

Each of the hydraulic displacement units 20 and 22 is shown in detail inFIG. 3 and is of generally the same construction. The hydraulicdisplacement unit 20 has a rotatable cylinder block 45 connected by asplined connection 46 to a drive input shaft 47 having an internal endrotatable in a journal 47a positioned in a center section, indicatedgenerally at 48, of the hydrostatic transmission. The outboard end ofthe drive input shaft 47 is rotatably supported by the top housing part12 by means of a bearing 49. A lip seal 50 seals the shaft opening inthe top housing part 12.

The rotatable cylinder block 45 has a series of piston-receivingchambers, each of which movably mount a piston 51 of a relatively largediameter and with each of the pistons 51 being urged by an associatedspring 52 into following engagement with a swashplate structure. Thehydraulic displacement unit 20 has overcenter variable displacement,with this operation being achieved by angular adjustment of a swashplate54 which, as well known in the art, can have its angle varied from theclockwise position shown in FIG. 3 to an opposite extreme position in aknown manner and by manually operable structure, not shown. Theswashplate can pivot about a pivot axis in a counterclockwise directionand past a horizontal center position, as viewed in FIG. 3. Theswashplate 54, as known in the art, mounts a thrust plate 55 againstwhich the pistons abut and a bearing and bearing guide structurerotatably support the thrust plate 55 relative to the body of theswashplate.

Each of the piston-receiving chambers has a passage 57 opening to a faceof the rotatable cylinder block 45 for coaction with arcuate ports ofthe center section 48 which will be described subsequently.

The hydraulic displacement unit 22 is a fixed displacement unit and hasa rotatable cylinder block 58 with a plurality of piston-receivingchambers each movably mounting a piston 59 which is spring-urged by aspring 60 toward a swashplate 61. The swashplate 61 has a thrust plate62 against which an end of the pistons engages and a ball thrust bearing63 interposed between the thrust plate and the swashplate to rotatablymount the thrust plate.

The rotatable cylinder block 58 drives the drive output shaft 35 througha splined connection 64 therebetween.

An inner end of the drive output shaft 35 rotates within an opening 65in the center section 48 which may optionally receive a journal 66 and,if the journal is not used, the opening 65 is cylindrical as shown inFIG. 11. The outboard end of the drive output shaft 35 is sealed by alip seal 67 and with bearing structure disposed interiorly thereofincluding a ball bearing 68.

Each of the piston-receiving chambers of the rotatable cylinder block 58has a passage 69 opening to a face thereof which coact with arcuateports associated with a face of the center section 48 to be subsequentlydescribed.

Since the hydraulic displacement unit 22 is of a fixed displacement, theswashplate 61 need not be adjustably mounted and, therefore, can besupported by the common housing 10 against hydraulic forces exertedthrough the pistons 59. As seen in FIG. 3, the centerline of the driveoutput shaft 35 is located on the split line 16 of the housing parts 12and 14 and extends through a central opening 69 in the swashplate 61.The swashplate 61 spans the split line and support thereof against fluidforces is provided by the common housing at both sides of the splitline.

The foregoing description generally describes the integrated hydrostatictransaxle wherein the bottom housing part 14 provides a common sump forthe transaxle components as is evident in FIGS. 1 and 2 and also for thehydrostatic transmission as is evident from FIGS. 1 to 3.

The hydraulic displacement units 20 and 22 have their respectiverotatable cylinder blocks arranged with their axes of rotation generallyat right angles to each other. It is the primary function of the centersection 48 to provide communication between selected piston-receivingchambers of the respective cylinder blocks 45 and 58. In achieving thisprimary function, center section 48 has been uniquely designed tominimize costly machining operations and enable formation of the body ofthe center section by casting. Examples of such casting, withoutlimitation, are lost foam casting and die casting. The resultingmaterial of the cast body of the center section has a relatively highdegree of porosity as compared to a conventional machined center sectionfor a hydrostatic transmission and in order to assure any leakageproblem of high pressure fluid contained within a passage in the centersection, because of porosity, is confined within the common housing, thecenter section 48 has been constructed as a separate one-piece centersection which is positionable within the bottom housing part 14 as seenin FIG. 3. The one-piece center section 48 is generally L-shaped to havea pair of faces generally at right angles to each other with one planarface 72 coacting with a face of the rotatable cylinder block 45 of thevariable displacement unit 20 and a second planar face 73 coacting witha face of the rotatable cylinder block 58 of the hydraulic displacementunit 22. The center section body has two integral parts 74 and 75oriented to have the two parts define the legs of the L shape of thecenter section, with the part 74 having the planar face 72 and the part75 having the planar face 73.

The planar face 72 has a pair of arcuate ports 76 and 77 and the planarface 73 has a pair of arcuate ports 78 and 79, as seen in FIGS. 8 and10, respectively.

First and second straight, generally parallel passages 80 and 81 arecast into the center section body and function to intersect the arcuateports and place the arcuate ports in paired relation for fluidcommunication. The first passage 80 intersects with arcuate port 76 andarcuate port 78 to provide a first pair of ports in fluid communication.The second passage 81 intersects arcuate ports 77 and 79 and places themin paired fluid communication.

In operation of the integrated hydrostatic transaxle, one or the otherof the first and second fluid passages functions to deliver fluid underpressure from the variable displacement unit 20 functioning as a pump tothe fixed displacement unit 22, functioning as a motor, and with theother fluid passage providing for return of fluid from the motor to thepump. The first and second fluid passages 80 and 81 terminate at one endat their intersection with the arcuate ports 78 and 79 and are closed attheir other end as formed in the casting process.

The center section 48 has a third passage 84 intersecting said firstpassage 80 and a fourth passage 85 intersecting the second passage 81,with the passages 84 and 85 opening to a surface 86 of the centersection opposite to the planar face 72.

A through bore 87 extends perpendicular to and is positioned between thefirst and second fluid passages 80 and 81 and a fifth fluid passage 88,sealed intermediate its ends by journal 47a, extends generally parallelto the through bore 87 and is positioned between the first and secondfluid passages 80 and 81. A sixth fluid passage 90 extends between andnormal to the fifth fluid passage 88 and a recess 91 in the centersection set back from the surface 86 of the center section.

The utility of the through bore and third through sixth passages will bereadily understood by reference to FIGS. 3 to 7 and the followingdescription.

The third and fourth fluid passages 84 and 85 mount a pair of checkvalves which each having a tubular seat member 93 and 94, respectively,fitted therein and which form seats for a pair of check valve balls 95and 96 spring-urged downwardly against the seats. The check valvesfunction, when closed, to block fluid flow from either of the first andsecond passages 80 and 81 to a recess or well 100 (FIG. 3) formed by acavity in the bottom housing port 14. This recess is generally oval andis defined by a continuous upstanding wall on the bottom housing partwith wall sections shown at 101 and 102. The lower ends of the third andfourth passages 84 and 85 open into this generally oval recess. The ovalrecess 100 is sealed off, at its top, by a generally oval-shaped wall103 on the underside of the center section 48 and which has a sealingO-ring 104 therebetween. This is a sealed recess or well so thatfiltered fluid in the recess may be a source of make-up fluid to thehydrostatic transmission. Structure associated with the check valvesalso provides for a bypass function wherein, even though the pump is setat a displacment and is operable, there is no drive of the motor sincethe first and second passages 80 and 81 are cross-connected throughopening of the check valves and the generally oval recess 100.

The make-up fluid is delivered to the generally oval recess 100 from thecommon sump within the bottom housing part 14 by flow through an openspace beneath the center section 48 (FIG. 3) and through a cylindricalfilter 110 having O-ring seals at its top and bottom. The interior ofthe filter 110 communicates with the sixth fluid passage 90 in thecenter section. As previously described, the sixth fluid passage 90communicates with the fifth fluid passage 88 and the fifth fluid passage88 communicates with the through bore 87 so that fluid reaches therecess 100.

The center section has a series of through mounting holes at 115, 116,and 117 whereby, as seen in FIG. 3, in assembly, the center section 48can be secured to the upper housing part 12, as by self-tapping screws118 and the final assembly achieved by bringing the bottom housing part14 into association with the top housing part 12 along the split line16.

All of the first through sixth fluid passages of the center section aswell as the through bore 87, recess 65, recess 91 and through mountingholes 115-117 can be formed in the center section in a casting process.There is only a limited amount of machining required to finish thecenter section. As previously stated, a cast center section has a higherporosity than a conventional machined center section, which could createthe possibility of leakage from whichever of the first and secondpassages 80 and 81 may have pressure fluid therein; however, theone-piece, integral center section which is independent of the housingsavoids any problem from such leakage since such leakage would merely beinto the common sump of the integrated hydrostatic transaxle and whichis open to atmosphere through a bleed tube 140.

The bypass operation previously referred to is effected by opening thecheck valves by raising the check valve balls 95 and 96 off their seats.The structure for this includes a bypass actuator structure including abypass actuator plate 120 and a bypass rod 121. The bypass actuatorplate 120, as seen in FIGS. 4 and 7, is positioned in the generally ovalrecess 100 in the bottom housing part 14 and, at its middle, isconnected to the lower end of the bypass rod 121 and has a pair ofupturned ends (FIG. 7) positioned beneath the check valve balls 95 and96. Lifting of the bypass rod 121 causes the bypass actuator plate tolift the check valve balls and place the center section first and secondpassages 80 and 81 in fluid communication. Lifting of the bypass rod 121is achieved by rotation of a handle 125 positioned above top housingpart 12 and, as seen particularly in FIGS. 2, 3 and 5. The bypass rod121 is longitudinally movable in an opening 126 in the top housing part12 as well as having its lower part extending downwardly through thethrough bore 87 of the center section and is normally urged downwardlyby a spring 127. As seen in FIG. 6, the handle 125 has cam shapes 130formed thereon which coact with ends of a through pin 131 fitted into anend of the bypass rod 121. Rotation of the handle 125 from the positionshown in the drawings to bring the cams 130 under the through pin 131raises the through pin and the bypass rod 121 to establish the bypassoperation.

The bypass rod 121 and center section 48 are uniquely associated withthe housing structure whereby a bypass operation also results inbleeding air from the system fluid. When the bypass rod 121 is in itslower position and the check valves are closed, the upper end of thethrough bore 87 of the center section 48 is closed by a seal washer 135backed up by peripheral flange on the bypass rod, so that there is nofluid communication between the through bore 87 and the interior of thecommon housing 10. When the bypass rod 121 is raised to effect a bypassoperation, the seal washer 135 is moved upwardly from its seat wherebythe upper end of the through bore 87 is open to the interior of thecommon housing and air can bleed off to the housing interior. Air thataccumulates in the common sump can bleed off to atmosphere through thebleed tube 140 (FIG. 1).

It is believed that the operation of the integrated hydrostatictransaxle is clearly apparent from the foregoing description. However,it may be briefly summarized as follows. An engine drives the driveinput shaft 47 for the variable displacement unit 20 (functioning as apump) to cause operation of the displacement unit 22 (functioning as amotor) and the drive output shaft 35 drives the transaxle componentsshown in FIG. 2 for rotation of the wheel axles 23 and 24. The directionof rotation of the wheel axles can be shifted from forward to reverse byshifting the swashplate 54 of the variable displacement unit 20 to aposition opposite side of center from that shown in FIG. 3 and withresulting reversal of pressure fluid flow through the center section 48from the pump to the motor. In the event there is to be no rotation ofthe wheel axles 23 and 24 while the pump is still operating and set fordisplacement, a bypass operation is achieved by rotation of the handle125 to raise the bypass rod 121 and open the check valve balls 95 and96. As previously mentioned, any air in the passages in the centersection can bleed to the sump of the common housing. Either one of thecheck valves can automatically open to provide make-up fluid to thetransmission circuit from the generally oval recess 110 when thepressure existing in one or the other of the first and second straightpassages 80 and 81 in the center section is sufficiently less than thatof the fluid in the oval recess to overcome the spring closing force ona check valve ball.

We claim:
 1. In combination, a hydrostatic transmission comprising apair of hydraulic displacement units, each having a rotatable cylinderblock with reciprocal pistons, and a housing for said displacement unitsproviding a fluid sump, said rotatable cylinder blocks having their axesof rotation generally normal to each other, a one-piece generallyL-shaped center section positionable in said housing and having firstand second face generally at right angles to each other, said centersection being positioned to have said first face engage an end of onerotatable cylinder block and the second face engage an end of the otherrotatable cylinder block, arcuate fluid ports at the first and secondfaces of said center section for coaction with said rotatable cylinderblocks, a first straight fluid passage in said center section connectingone of the ports at each face to define a first pair of fluidcommunicating ports and terminating at one of said pair of ports, and asecond straight fluid passage in said center section connecting anotherof the ports on each face to define a second pair of fluid communicatingports and terminating at one of the ports of said second pair.
 2. Thecombination of claim 1 wherein said L-shape of the one-piece generallyL-shaped center section has two legs and two integral parts of thecenter section are oriented to have the two parts define said legs ofthe L-shape, and each part has one of said faces.
 3. The combination ofclaim 1 wherein said one-piece generally L-shaped center section is ofmaterial which may be sufficiently porous to permit leakage of highpressure fluid from whichever one of said straight fluid passagescontains high pressure fluid, and the one-piece generally L-shapedcenter section being separate from the housing enabling said leakage toflow to said fluid sump.
 4. The combination of claim 1 wherein saidone-piece generally L-shaped center section has third and fourthstraight fluid passages intersecting said first and second fluidpassages, respectively, and opening to a surface of said center sectionopposite to one of the faces thereof.
 5. The combination of claim 4wherein said first and second fluid passages are generally parallel,said one-piece generally L-shaped center section has a through boreextending perpendicular to and positioned between said first and secondfluid passages, a fifth fluid passage extending generally parallel toand positioned between said first and second fluid passages and openingto said bore, and a sixth fluid passage extending between the fifthfluid passage and a recess set back from the surface to which the thirdand fourth fluid passages open.
 6. An integrated hydrostatic transaxlehaving a common housing for a hydrostatic transmission and a pair ofoppositely-extending differentially-connected axles to provide a commonsump, said hydrostatic transmission comprising a pair of hydraulicdisplacement units, each having a rotatable cylinder block withreciprocal pistons with at least one of said units being an over-centervariable displacement unit to provide forward and reverse drive of saidaxles, said rotatable cylinder blocks having their axes of rotationgenerally normal to each other, a one-piece generally L-shaped centersection separate from said housing and having first and second facesgenerally at right angles to each other, said center section beingpositioned to have said first face engage an end of one rotatablecylinder block and the second face engage an end of the other rotatablecylinder block, arcuate fluid ports at the first and second faces ofsaid center section, and first and second straight and generallyparallel fluid passages intersecting and connecting an arcuate fluidport at each face in pairs whereby said first fluid passage may providedelivery of high pressure fluid from one fluid port of a pair to theother fluid port of said pair in one direction of drive of the axles andthe second fluid passage may provide delivery of high pressure fluidfrom one fluid port of the other pair to the other fluid port of saidother pair in an opposite direction of drive of the axles.
 7. Anintegrated hydrostatic transaxle as defined in claim 6 wherein saidcenter section is of a material having a porosity which may permitleakage from one of said first and second fluid passages to said commonsump.
 8. An integrated hydrostatic transaxle as defined in claim 6wherein said center section has a cast body.
 9. An integratedhydrostatic transaxle as defined in claim 6 wherein said one-piecegenerally L-shaped die-cast center section has third and fourth straightfluid passages intersecting said first and second fluid passages,respectively, and opening to a surface of said center section oppositeto one of the faces.
 10. An integrated hydrostatic transaxle as definedin claim 9 wherein said first and second fluid passages are generallyparallel, said one-piece generally L-shaped die-cast center section hasa through bore extending perpendicular to and positioned between saidfirst and second fluid passages, a fifth fluid passage extendinggenerally parallel to and positioned between said first and second fluidpassages and opening to said bore, and a sixth fluid passage extendingbetween the fifth fluid passage and a recess set back from the surfaceto which the third and fourth fluid passages open.
 11. An integratedhydrostatic transaxle having a common housing for a hydrostatictransmission and a pair of oppositely-extending differentially-connectedaxles to provide a common sump, said hydrostatic transmission comprisinga pair of hydraulic displacement units, each having a rotatable cylinderblock with reciprocal pistons with at least one of said units being anover-center variable displacement unit to provide forward and reversedrive of said axles, said rotatable cylinder blocks having their axes ofrotation normal to each other, a one-piece L-shaped center sectionseparate from said housing and having first and second faces at rightangles to each other, said center section being positioned to have saidfirst face engage an end of one rotatable cylinder block and the secondface engage an end of the other rotatable cylinder block, said centersection having integrally-formed arcuate fluid ports on said first andsecond faces, first and second straight and generally parallel fluidpassages intersecting and connecting an arcuate fluid port at each facein pairs whereby said first fluid passage may provide delivery of highpressure fluid from one fluid port of a pair to the other fluid port ofsaid pair in one direction of drive of the axles and the second fluidpassage may provide delivery of high pressure fluid from one fluid portof the other pair to the other fluid port of said other pair in anopposite direction of drive of the axles, said center section being of amaterial of a porosity which may permit leakage of high pressure fluidfrom one of said fluid passages to said common sump, and each of saidfirst and second fluid passages terminating at one of said ports. 12.The integrated hydrostatic transaxle of claim 11 wherein said L-shape ofthe one-piece L-shaped center section has two legs and two parts of thecenter section are oriented to have the two parts define said legs ofthe L-shape, and each part has one of said faces, said one-pieceL-shaped center section has third and fourth straight fluid passagesintersecting said first and second fluid passages, respectively, andopening to a surface of said center section opposite to one of the facesthereof having arcuate fluid ports, said first and second fluid passagesbeing generally parallel, a through bore in said one-piece L-shapedcenter section extending perpendicular to and positioned between saidfirst and second fluid passages, a fifth fluid passage extendinggenerally parallel to and positioned between said first and second fluidpassages and opening to said bore, and a sixth fluid passage extendingbetween the fifth fluid passage and a recess set back from the surfaceto which the third and fourth fluid passages open.
 13. In combination, ahydrostatic transmission comprising a pair of hydraulic displacementunits, each having a rotatable cylinder block with reciprocal pistons,and a housing for said displacement units providing a fluid sump, saidrotatable cylinder blocks having their axes of rotation normal to eachother, a one-piece L-shaped center section separate from said housingand having first and second faces at right angles to each other, saidcenter section being positioned to have said first face engage an end ofone rotatable cylinder block and the second face engage an end of theother rotatable cylinder block, each of said center section faces havingarcuate fluid ports for coaction with a rotatable cylinder block, afirst straight fluid passage in said center section connecting one ofthe ports on each face and terminating at one of said ports, a secondstraight fluid passage in said center section connecting another of theports on each face and terminating at one of said ports, said centersection being of material which may be sufficiently porous to permitleakage of high pressure fluid from whichever one of said straight fluidpassages contains high pressure fluid with said leakage flowing to saidfluid sump, and said center section having third and fourth straightfluid passages intersecting said first and second fluid passages,respectively, and opening to a surface of said center section oppositeto one of the faces thereof for mounting of check valves.
 14. Thecombination of claim 13 wherein said first and second fluid passages aregenerally parallel, said center section has a through bore extendingperpendicular to and positioned between said first and second fluidpassages, a fifth fluid passage extending generally parallel to andpositioned between said first and second fluid passages and opening tosaid bore for delivery of make-up fluid to said bore, and a sixth fluidpassage extending between the fifth fluid passage and a recess set backfrom the surface to which the third and fourth fluid passages open forcommunication with a source of filtered make-up fluid.